Index getting/setting

Julia Arrays are treated differently from other AbstractArrays. This results in surprising, inconsistent behavior.

* Arrays are automatically converted to numpy arrays. So indexing into them works, and they are 0-indexed.
* Other AbstractArrays, such as DataFrames, are not converted. So indexing into them doesn't work, and even if it did, they would be 1-indexed. This is rather strange.

Would it be possible to overload indexing for array-like operators, and to handle the 0 -> 1 indexing conversion automatically?

```
In [1]: import julia

In [2]: j = julia.Julia()

In [3]: A = j.eval("zeros(3,3)")
...: A[1,1] = 2
...: A
...:
Out[3]:
array([[ 0., 0., 0.],
[ 0., 2., 0.],
[ 0., 0., 0.]])

In [4]: j.using("DataFrames")
...: Adf = j.DataFrame(A)
...: Adf[1,1] = 2
...:
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
in ()
1 j.using("DataFrames")
2 Adf = j.DataFrame(A)
----> 3 Adf[1,1] = 2

TypeError: 'PyCall.jlwrap' object does not support item assignment
```